The present invention relates to the manufacture of integrated circuits. Specifically, a system for heating semiconductor substrates in a controlled pressure and temperature environment is disclosed.
The manufacture of integrated circuits, such as metal oxide semiconductors (MOS), requires rapid thermal processing of semiconductor wafers in a controlled pressure environment, such as vacuum. For instance, in the process of forming MOS transistors, the gate oxide layer is typically formed by thermal oxidation of a silicon substrate in a substantially pure oxygen atmosphere. However, in certain applications such as MOS ULSI circuits, the gate oxide layers can exhibit undesirable characteristics, such as relatively high defect densities and charge trapping, along with relatively low reliability and resistance problems due to hot carrier effects.
It is known that the gate dielectric characteristics of MOS transistors can be improved using a sequence of rapid thermal processing (RTP) of the silicon substrate. These processing steps include: (1) creating an oxynitride growth with nitric oxide (NO); (2) applying silicon nitride (SiN) with a chemical vapor deposition (CVD) process; (3) annealing with ammonia (NH3); and (4) annealing with N2O. The various RTP processing steps are conducted generally in a vacuum capable chamber with a controlled pressure/vacuum and a controlled temperature. An RTP oven is partitioned with quartz windows defining a central vacuum chamber that holds a wafer to be heated by multiple arrays of radiant heating lamps. The quartz windows separate the wafers from heating lamps and other sources of contaminants during the heating process. The edges of the quartz windows are sealed with the chamber walls to form an air-tight chamber enclosure. When a vacuum is drawn in the chamber, an atmospheric force between two and four tons is produced against the quartz windows. The quartz windows are thick enough to withstand this force, and are generally at least about 25 mm to 35 mm thick. Thinner quartz windows, generally at least about 3 mm to 6 mm thick, are used only for chambers that operate at atmospheric pressures.
The quartz window isolation chamber structure, while maintaining the inner chamber environment clean of contaminants, introduces a large thermal mass between the heating source (lamps) and the wafer within the chamber, making heating less efficient and wafer temperature control more difficult. The additional thermal mass makes it difficult to maintain process repeatability and quality control. The quartz windows, due to their thickness, are subject to breakage, and add significant cost to the RTP apparatus. Accordingly, a system for rapid thermal processing which avoids the complications, expense, and repeatability problems created by quartz window-based ovens would be desirable.
Moreover, efforts to increase throughput for semiconductor wafer RTP processing have yielded certain alternatives other than lamp-based heating. Mattson Technology offers an ASPEN II RTP system that processes two wafers in a single process chamber using susceptor-based heating. U.S. Pat. No. 6,133,550 discloses a method for RTP processing wafers by rapidly inserting and removing them from a furnace. Increasing wafer size and increasing stresses on larger and larger chamber windows for chambers to accommodate larger wafers have limited the potential for increasing throughput for lamp-based RTP systems by processing multiple wafers in a chamber. Accordingly, a system for lamp-based rapid thermal processing that permits increased wafer throughput would also be desirable.
Rapid thermal processing has been used to anneal semiconductor substrates following ion implantation. In a typical rapid spike anneal, the substrate is heated to 1100xc2x0 C. (in the case of silicon semiconductor wafers) at a rate of from 200xc2x0 to 300xc2x0 per second. Once the surface has reached the peak anneal temperature, it is cooled at rates on the order of 80xc2x0 C. per second. The slower the rate of cooling, the greater the migration of the implanted species following the anneal. For example, a bare silicon wafer implanted with boron (BF2+) to a depth of about 240 xc3x85 and spike annealed at a maximum temperature of 1050xc2x0 C. exhibits migration of the junction depth as a function of the ramp down rate used following the spike temperature. When cooled at a rate of between 80xc2x0 C. to 120xc2x0 C. per second, the junction depth increases to over 300 xc3x85. As shallower and shallower junction depths are sought, the adverse effects of junction depth migration during treatment become more and more critical.
One factor limiting the rate at which the temperature of the semiconductor substrate surface may be cooled is the residual heating as a result of lamp filament emitting radiation following lamp shut off, which residual radiation can pass through the lamp bulb envelope or enclosure and into the chamber and reach the substrate. Because the lamp filament remains hot for fraction of a second (generally 100 to 1000 msec), the radiation still emitted from the filament delays the start of controlled cool down or ramp down. Accordingly, a better way to expedite temperature ramp down following heating in rapid thermal processing is sought.
The rapid thermal processing (RTP) system according to the invention provides a controlled pressure and temperature environment for processing substrates, such as semiconductor wafers and integrated circuits. The apparatus includes a heating chamber and an array of heat lamps that generate radiant heat for maintaining the temperature of a semiconductor wafer held within the chamber at a selected value or range of values according to a desired heating recipe. Each heat lamp includes a bulb, and at least such bulb is surrounded by an optically transparent enclosure that isolates the bulb from the interior of the chamber and the wafer therein. Preferably, the optically transparent enclosure is formed from quartz and has a surface completely or substantially transparent to the radiant heat energy emitted by the bulb. By isolating the chamber interior and the wafer therein from the bulb and associated components of the heating lamp, the optically transparent enclosure helps prevent contaminants from the heating lamps from entering the chamber or being deposited on a semiconductor wafer in the chamber.
In another aspect of the invention, improved temperature control is realized by using heat lamps with bulbs having a reflector surface disposed over at least a portion of the bulb surface or disposed over at least a portion of the optically transparent enclosure. The reflectors help to control and direct radiation from the lamps to the surface of a semiconductor wafer under process. Alternatively, the reflector surface may be found on the wall of the chamber, particularly within a cavity in the chamber wall with a concavely-shaped or parabolic-shaped inner surface. When the heat lamps are positioned within the cavity, the reflector surface on the cavity wall helps to control and direct radiation from the lamps to the surface of a semiconductor wafer under process.
In yet another aspect of the invention, an apparatus for rapid thermal processing of a semiconductor substrate, such as a semiconductor wafer, or a system for processing one or multiple semiconductor substrates, has a chamber defining a volume into which the semiconductor substrate is introduced for heating by one or more heating lamps. Preferably each heating lamp is isolated from the chamber volume by an optically transparent envelope that has an associated reflector. The envelope may be rotated about its longitudinal axis from a first position in which the reflector directs radiant heat energy emitted by the heating lamp toward the substrate to a second position wherein the reflector shields the substrate from a portion or all of the radiant heat emitted by the heating lamp. The rotation of the envelope may be accomplished independently or together with rotation of the lamp bulb. Preferably, the second position of the reflector is where the optically transparent enclosure has been rotated from 120 degrees to 180 degrees either clockwise or counter-clockwise about its longitudinal axis.
Preferably, the optically transparent enclosure forms a generally cylindrical tube and the reflector associated with the optically transparent envelope is a reflective coating applied over the internal surface of the optically transparent envelope in an arc of from about 90 degrees to about 180 degrees. Optionally, the reflective coating may be applied over the internal surface of the optically transparent envelope to form an elliptic arc with a substantially uniform coating thickness along the arc. Preferred reflective coating materials for coating the envelope include, but are not limited to metallic gold or other infrared reflective coatings, such as TiO2.
Preferably, the means for rotating the optically transparent envelope are sleeve or ball bearings and an associated motor. For example, a first sleeve with associated ball bearings may be mounted at the first end of the envelope and a second sleeve with associated ball bearings may be mounted at the second end of the envelope, and a motor may be operatively connected thereto.
Alternatively, the reflector associated with the optically transparent envelope is a reflective coating applied to an external surface of the heating lamp, preferably in an arc of from about 90 degrees to about 120 degrees. Optionally, the reflective coating is applied in a substantially uniform coating thickness. Preferred reflective coating materials for coating the surface of the heating lamp bulb include, but are not limited to metallic gold or other infrared reflective coatings, such as TiO2. In this alternative embodiment, the lamp may be rotated about its longitudinal axis from a first position where the reflector directs radiant energy toward the substrate to a second position from 120 degrees to 180 degrees either clockwise or counter-clockwise, wherein the reflector shields the substrate from a portion or all of the radiant energy. In such alternative embodiment, the means for rotating are associated with the lamp, or with both the lamp and the optically transparent envelope or enclosure.
Still another aspect of the invention is a method for rapid thermal processing of a semiconductor substrate or multiple semiconductor substrates, such as one or more semiconductor wafers. In the method, a portion or all of the radiant energy emitted by a heating lamp is alternately reflected toward and shielded away from the substrate held within a chamber volume with a reflector associated with an optically transparent envelope, which envelope isolates the heating lamp from the substrate in the chamber volume. The reflector reflects radiant energy when the reflector is in a first position and the reflector shields a portion or all of the radiant energy from the substrate when the reflector is in a second position.
Preferably, the optically transparent enclosure forms a generally cylindrical tube and the reflector associated with the optically transparent envelope is a reflective coating applied over the internal surface of the optically transparent envelope in an arc of from about 90 degrees to about 180 degrees. Preferably, the reflective coating is a material such as metallic gold, or other infrared reflective coatings, such as TiO2. Where the reflector is a reflective coating on the optically transparent enclosure or envelope around the lamp bulb, shielding is by rotating the optically transparent enclosure about its longitudinal axis from a first position to a second position that is from 120 degrees to 180 degrees of clockwise or counter-clockwise rotation.
Alternatively, the reflector associated with the optically transparent envelope is a reflective coating applied to an external surface of the heating lamp, preferably in an arc of from about 90 degrees to about 120 degrees. Preferred reflective coating materials for coating the surface of the heating lamp bulb include, but are not limited to metallic gold or other infrared reflective coatings, such as TiO2. In this alternative embodiment, the lamp may be rotated about its longitudinal axis from a first position where the reflector directs radiant energy toward the substrate to a second position from 120 degrees to 180 degrees either clockwise or counter-clockwise, wherein the reflector shields the substrate from a portion or all of the radiant energy. In such alternative embodiment, the means for rotating are associated with the lamp, or with both the lamp and the optically transparent envelope or enclosure.
Shielding radiation from the substrate by rotating the optically transparent enclosure with the reflector, the lamp with reflector or both the lamp and the optically transparent enclosure with reflector may be partial or complete, depending upon the extent to which the reflector shields the substrate from radiation. The method may be used to controllably cool a substrate after the substrate has reached its peak heating temperature. Thus, the heating lamp may be deactivated before rotating lamp or enclosure with associated reflector to shield the substrate from radiation. Alternatively, the lamp or enclosure with associated reflector may be rotated into position for shielding radiation before lamp power has been switched off. Preferably, the substrate is shielded from radiation by placing the reflector in the second or shielding position within xe2x88x92[minus] 100 msec before to +[plus] 500 msec after the substrate has reached its desired peak surface temperature.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.